The endocannabinoid system (ECS) is a complex neuromodulatory network involved in maintaining physiological balance through interactions with various neurotransmitter and hormonal pathways. Its key components—cannabinoid receptors (CBRs)—are activated by endogenous ligands and exogenous cannabinoids such as those found in the Cannabis sativa plant. Although cannabinoids like cannabidiol (CBD) have garnered interest for their potential therapeutic effects, evidence regarding their safety, particularly for reproductive health, remains limited. This review summarizes the structure and molecular mechanisms of the ECS, its role in reproductive physiology—including its interactions with the hypothalamic–pituitary–gonadal axis (HPG axis), gametogenesis, implantation, and lactation—and the possible consequences of cannabinoid exposure for fertility. In addition, we focus on the involvement of the ECS and cannabinoids in breast cancer, highlighting emerging evidence on their dual role in tumor progression and therapy. These insights emphasize the need for further research to better define the therapeutic potential and risks associated with cannabinoid use in reproductive health and breast cancer.

Background/Objectives: The corpus luteum (CL) in the dog is the sole source of progesterone (P4) during diestrus and pregnancy, making it a key regulator of reproductive function. However, robust and functionally validated in vitro models of canine luteal cells remain limited. This study aimed to establish and characterize a reproducible primary culture system of canine luteal cells with preserved steroidogenic activity and regulatory responsiveness. Methods: Ovaries containing CLs were collected from five clinically healthy bitches undergoing routine ovariohysterectomy (OHE). Luteal tissue was mechanically fragmented and enzymatically digested using collagenase type II. Primary cultures were established using an explant-based approach and maintained in Dulbecco’s Modified Eagle Medium/Ham’s F-12 nutrient mixture (DMEM/F12) or Roswell Park Memorial Institute medium 1640 (RPMI 1640) supplemented with 20% fetal bovine serum (FBS). Cellular morphology, proliferation, expression of steroidogenic markers—steroidogenic acute regulatory protein (STAR) and 3β-hydroxysteroid dehydrogenase type 1 (HSD3B1), P4 secretion, and responsiveness to forskolin stimulation were evaluated. Results: Cultured luteal cells exhibited stable attachment, proliferation, and a predominantly spindle-shaped morphology. Both media supported maintenance of a steroidogenic phenotype, while RPMI 1640 enabled enhanced proliferation, allowing expansion up to passage three and efficient cryobanking. Cells remained functionally active, secreting progesterone for up to 28 days in vitro. Forskolin stimulation increased progesterone secretion up to 2.7-fold, confirming preserved cyclic AMP-dependent steroidogenic responsiveness. Conclusions: The canine CL is a reliable source of functionally competent luteal cells, and the established culture system represents a physiologically relevant in vitro model. To our knowledge, this is the first functionally validated in vitro model of the canine CL. This platform enables controlled investigations of luteal function, endocrine regulation, and mechanisms of P4 synthesis, supporting its application in mechanistic and translational reproductive research.